A PCCR-based approach to determining the gaze of an eye may use an image of the eye either in its bright-pupil condition (a retinal retro-reflection complements the iris image) or dark-pupil condition (a cornea-scleral reflection complements the iris image). In a given situation, the respective images may be of different quality, and it may even be impossible to carry out an uninterrupted gaze tracking based on just one of these imaging modes. Therefore, to be able to choose the optimal mode, some available eye trackers comprise double reference illuminators for creating the reflections. A first reference illuminator, for use in imaging in the bright-pupil mode, is then arranged coaxially with the optic axis of a camera (image sensor), whereas a second reference illuminator, for use in the dark-pupil mode, is arranged off the camera axis. Such a reference illuminator may be a compound light source arranged round the camera objective in a concentric ring; cf. FIG. 3 in Applicant's patent SE 524003.
It is known in the art (see, e.g., the paper General Theory of Remote Gaze Estimation Using the Pupil Center and Corneal Reflections by E. D. Guestrin and M. Eizenmann, IEEE Transactions on Biomedical Engineering, Vol. 53, No. 6, pp. 1124-1133 (June 2006), included herein by reference) that the eye's position and orientation, at a given point in time, cannot be unambiguously determined unless the locations of two distinct corneal reflection (or glints, or first Purkinje reflections) can be extracted from one image of the eye or from several, simultaneous images. If two reference illuminators are used simultaneously, however, coexisting glints will mutually blur the measurements by reflections and the like. If the reference illuminators are used alternately (e.g., by time interlacing), then a small time delay will necessarily separate the two images, to the detriment of the accuracy, particularly if the delay falls in the duration of a saccade. The delay also makes the eye tracking slower. A similar drawback becomes noticeable if the bright-pupil image is used for providing an initial guess in the process of finding the location of the pupil centre in the dark-pupil image. This is practiced in the art, as described, notably, in patent application US 2004/0005083. Since the two images cannot be acquired simultaneously, such initial guess is sometimes of little avail.
As many of those skilled in the art will acknowledge, the accuracy of eye tracking is highly dependent on the resolution of the camera used for imaging the eye with the glints. Indeed, the virtual image of the reference illuminator formed by reflection in the cornea is shrunk by a factor 100 or more (assuming a corneal focal length of 4 mm and an illuminator-to-eye distance of at least 400 mm). On the other hand, to avoid serious round-off errors, the image of the reference illuminator should occupy a region of at least, say, ten camera pixels. Hence, for an eye tracker to be useful, a reasonably high performance is required from the camera, which therefore defines a least possible price of the product.
Conventional eye trackers generally perform optimally if the studied person does not move during a measurement session. Particularly annoying are head movements that change the angle between the head and the camera of the eye tracker, because this may introduce obscuring objects into the line of sight from the reference illuminator to the eye or into the line from the eye to the camera. Notably, spectacle frames, eyelashes, eyebrows, nose and protruding brow bones may cause problems of this kind.
It is probably similar considerations that have led to the widespread use of ring-shaped reference illuminators in eye trackers. Conventionally there is a larger ring for providing off-axis illumination and a smaller ring arranged around the circumference of the camera objective to be as coaxial as possible. By surrounding all sides of the camera objective with luminous points, the risk of having the tracked eye obscured is decreased. However, a ring-shaped illuminator is imaged in the cornea as an inhomogeneous spot having lower luminance than a solid light source would. This is detrimental to image contrast and makes it more difficult to find the location of the reflection of the light source. The problem is most severe in the case of the coaxial, smaller illuminator, which is further shrunk by reflection in the convex cornea, as seen above.
In view of the above shortcomings associated with available eye trackers, there appears to be a need for improved eye-tracking devices as regards accuracy, speed, reliability and cost efficiency.